1. Field of the Invention
The present invention relates to a design of a structure of a flexible circuit board, and in particular to a planarized cover layer structure of a flexible circuit board.
2. The Related Arts
In various electronic products that are currently prevailing, including notebook computers, personal digital assistants, and mobile phones, flexible flat cable and flexible circuit boards are commonly used as a carrier board for transmission of signals.
Referring to FIGS. 1 and 2, which are respectively a schematic plan view and a cross-sectional view showing a conventional flexible circuit board, as shown in the drawings, a substrate 1 has a first surface 1 and a second surface 2.
A conductor layer 2 is bonded to the first surface 11 of the substrate 1. The conductor layer 2 comprises a plurality of extended conductive signal lines 21. The conductive signal lines 21 are spaced from each other by a predetermined distance, whereby every two adjacent ones of the conductive signal lines 21 define therebetween a separation area 22. The conductive signal lines 21 have a predetermined line height h1 and are generally made of a copper foil material or a composite material.
The substrate 1 is arranged to extend in an extension direction and a free end of the substrate 1 is provided with a plurality of conductive contacts 13. The free end of the substrate 1 is insertable into an insertion slot (not shown) to have the conductive contacts 13 of the substrate 1 electrically engaging corresponding contacts provided in the insertion slot. A first adhesive layer 31 is formed on a surface of the conductor layer 2, which is bonded by the first adhesive layer 31 to an insulation cover layer 4.
In a known manufacturing process, the insulation cover layer 4 is subjected to pressing so that with the adherence of the first adhesive layer 31, the insulation cover layer 4 is securely bonded to the first surface 11 of the conductor layer 2. Due to the separation areas 22 existing between the conductive signal lines 21 of the conductor layer 2, during the process when the insulation cover layer 4 is pressed to bond, the first adhesive layer 31 is deformed and recessed due to being subject to the pressure thereby showing a wave like corrugated surface structure. Thus, the surface where the insulation layer 4 is set is irregular and non-flat surface. This might lead to poor quality of the flexible circuit board and inconsistency of electrical impedance among signal transmission paths provided on the flexible circuit board may result, making it difficult to precisely control the electrical impedance thereof.
Referring to FIG. 3, a cross-sectional view is given to illustrate another conventional flexible circuit board, of which the structure is similar to the conventional flexible circuit board shown in FIG. 2. A difference is that each of the conductive signal lines 21 of the conductor layer 2 is bonded by a second adhesive layer 32 to the first surface 11 of the substrate 1.
In practical applications, both known flexible circuit boards exhibit the above discussed shortcoming and thus lead to problems, such as poor impedance match, reflection of signal, spreading of electromagnetic wave, errors in transmitting and receiving signals, and distortion of signal waveform. The existence of these problems causes troubles to the circuit boards that are used in commonly used high-precision electronic facility.
For electronic devices (such as notebook computers) that have relatively high working frequencies, the precision of impedance gets more severe for higher working frequencies. The circuit boards that are manufactured with the conventional technology may not suit the need of the industry.